This invention relates to a multilayer reflective film coated substrate, a manufacturing method thereof, a reflective mask blank, and a reflective mask. More specifically, this invention relates to a multilayer reflective film coated substrate having high surface smoothness with less defects which is suitable for a reflective mask for use in a lithography method using exposure light in a short wavelength region such as extreme ultraviolet light, and further relates to a method of efficiently manufacturing such a multilayer reflective film coated substrate, a reflective mask blank obtained using such a multilayer reflective film coated substrate, and a high-quality reflective mask obtained using such a reflective mask blank, which is less defective and excellent in pattern transferability.
In recent years, following higher integration of semiconductor devices, patterns finer than the transfer limit of a photolithography method using the conventional ultraviolet light have been required in the semiconductor industry. In order to enable transfer of such fine patterns, the extreme ultraviolet (EUV) lithography being an exposure technique using EUV light with a shorter wavelength has been expected to be promising. The EUV light represents light in a wavelength band of the soft X-ray region or the vacuum ultraviolet ray region and, specifically, light having a wavelength of approximately 0.2 to 100 nm. As an exposure mask for use in the EUV lithography, a reflective mask as described in Japanese Unexamined Patent Application Publication (JP-A) No. H08-213303 has been proposed.
Such a reflective mask has a structure in which a multilayer reflective film for reflecting the EUV light serving as exposure light is formed on a substrate and, further, an absorber film for absorbing the EUV light is formed in a pattern on the multilayer reflective film. When pattern transfer is carried out using an exposure apparatus (pattern transfer apparatus) with the reflective mask disposed therein, the exposure light incident on the reflective mask is absorbed at a portion where the absorber film pattern is present, while, is reflected by the multilayer reflective film at a portion where the absorber film pattern is not present so that the reflected light is transferred onto, for example, a semiconductor substrate (resist-coated silicon wafer) through a reflective optical system.
As the foregoing multilayer reflective film, use is normally made of a multilayer film in which a material having a relatively high refractive index and a material having a relatively low refractive index are alternately layered in the order of several nm. For example, a multilayer film having Si and Mo thin films alternately layered is known as having a high reflectance with respect to EUV light of 13 to 14 nm.
The multilayer reflective film can be formed on the substrate, for example, by ion beam sputtering. In the case of containing Si and Mo, a Si target and a Mo target are used to alternately carry out sputtering so as to laminate Si and Mo films by approximately 30 to 60 cycles, preferably by approximately 40 cycles.
In order to further increase the reflectance with respect to the EUV light, it is necessary to reduce the surface roughness of the multilayer reflective film, while, the surface roughness of the multilayer reflective film largely depends on the surface roughness of the substrate. When, for example, the formation of the multilayer reflective film is carried out by inclining the substrate (i.e. oblique incidence film formation) for equalizing in-plane film thickness distribution of the Si and Mo thin films, this results in an increased surface roughness greater than the surface roughness of the substrate.
Therefore, the requirement for the surface roughness of the substrate is strict and, for example, it is reported that a root-mean-square (Rms) roughness of 0.10 nm or less is desirable.
A glass substrate is normally used as the foregoing substrate. However, even if a glass polishing method of the current state is applied, it is actually quite difficult to obtain a highly smooth and defectless surface that can satisfy the foregoing surface roughness requirement in the case of a glass having a multicomponent amorphous structure.
Generally, a multilayer reflective film is formed on a substrate according to an oblique incidence film formation method. Specifically, as shown in FIG. 1A, in the oblique incidence film formation method, film formation is carried out by disposing a target 40 and a substrate 1 so that sputtered particles from the target 40 are incident on the substrate 1 (the sputtered particles are scattered toward the substrate 1 as a particle group having an increasing width as indicated by hatching) from an oblique direction 41 with respect to a perpendicular direction S. The reason for using such an oblique incidence film formation method is that highly uniform in-plane film thickness distribution of the multilayer reflective film formed can be easily obtained. In contrast, as shown in FIG. 1B, there is a method of forming a multilayer reflective film by a normal incidence film formation method in which film formation is carried out by disposing a target 40 and a substrate 1 so that sputtered particles from the target 40 are incident on the substrate 1 (the sputtered particles are scattered toward the substrate 1 as a particle group having an increasing width as indicated by hatching) from a substantially perpendicular direction 42.
According to the study of the present inventors, when the multilayer reflective film is formed by the oblique incidence film formation method, there is the effect that the uniform in-plane film thickness distribution of the formed multilayer reflective film can be easily obtained as described above, while, there is an adverse effect of making larger a convex projection defect (hereinafter referred to as a “convex defect”) present on the surface of the substrate 1. Thus, as shown in FIG. 2A, even if a convex defect 7 on the substrate 1 is too small to be detected by a defect inspection apparatus, it may become a convex defect 2a, which is large enough to be a transfer pattern defect, on the surface of a multilayer reflective film 2 formed by the oblique incidence film formation method.
On the other hand, Japanese Unexamined Patent Application Publication (JP-A) No. 2003-515794 (Japanese translation of PCT international application) discloses to provide a multilayer buffer layer on a reticle substrate in order to reduce an adverse affect of a defect on the reticle substrate. The multilayer buffer layer is formed by the normal incidence film formation method. When the multilayer buffer layer is formed by the normal incidence film formation method, even if a convex defect present on the surface of the reticle substrate can be reduced in size to some extent, a concave defect present on the surface of the reticle substrate may increase in size. Therefore, it is not possible to correct both concave and convex defects solely by this film forming method. This is equivalently shown in FIG. 2B in which numeral 7 denotes a convex defect present on the surface of a substrate 1 while numeral 8 denotes a concave defect present on the surface of the substrate 1. Further, even if the convex defect 7 present on the surface of the substrate 1 is caused to disappear on the surface of a multilayer reflective film 2, since the reflection is the sum of reflections from respective layers of the multilayer reflective film 2, it is easily expected that a phase defect still occurs.